This application is related to Japanese Patent Applications No. 2000-220910 filed on Jul. 21, 2000, and No. 2001-120160 filed on Apr. 18, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a super-critical (trans-critical) heat pump in which a discharge pressure of refrigerant from a compressor exceeds the critical pressure of the refrigerant in a heat pump cycle for transferring heat in a low temperature side to a high temperature side, and is effectively applied to the one using carbon dioxide as a refrigerant.
2. Description of Related Art
In a vapor compressing cycle described in U.S. Pat. No. 5,890,370, a discharge pressure of refrigerant from a compressor exceeds the critical pressure of refrigerant, and a high-pressure side refrigerant pressure is controlled based on a refrigerant temperature at an outlet side of a radiator (high-pressure side heat exchanger). In this connection, when the inventors built a prototype of a heat pump cycle for heating hot water in a high-pressure side heat exchanger using the vapor compressing cycle, the following problem occurred.
That is, in this cycle, because hot water supplied for heating is repeatedly circulated between the high-pressure side heat exchanger and a heating heat exchanger, the temperature of the hot water flowing into the high-pressure side heat exchanger gradually increases. For this reason, a difference in temperature between the refrigerant and the hot water gradually decreases in the high-pressure side heat exchanger, so that the heat quantity (enthalpy) to be extracted from a heat pump gradually decreases and hence the coefficient of performance (COP) of the heat pump gradually deteriorates.
With respect to this problem, it is thought to take measures of increasing the high-pressure side refrigerant temperature (high-pressure side refrigerant pressure) to prevent a difference in temperature between the refrigerant and the hot water from decreasing. However, it is essential only that the temperature of the hot water is increased to a temperature enough for heating. When the high-pressure side refrigerant temperature is increased to ensure the difference in temperature between the refrigerant and the hot water, a refrigerant pressure is increased more than a necessary refrigerant pressure, and therefore, the power consumption of the compressor increases unnecessarily.
In view of the foregoing problems, it is an object of the present invention to provide a heat pump cycle which prevents the quantity of heat (enthalpy) to be obtained from a heat pump from decreasing and hence the coefficient of performance (COP) of the heat pump from becoming worse.
According to the present invention, a heat pump cycle includes a compressor for compressing refrigerant to a pressure higher than critical pressure of refrigerant, a first high-pressure side heat exchanger disposed to perform a heat exchange between refrigerant discharged from the compressor and a first fluid, a second high-pressure side heat exchanger disposed to perform a heat exchange between refrigerant flowing from the first high-pressure side heat exchanger and a second fluid having a temperature lower than that of the first fluid, a decompression unit disposed for controlling the pressure of refrigerant discharged from the compressor and for decompressing refrigerant flowing from the second high-pressure side heat exchanger, and a low-pressure side heat exchanger disposed to evaporate refrigerant decompressed in the decompression unit. Accordingly, a heat quantity obtained in a heat pump of the heat pump cycle becomes the sum of a heat quantity to be extracted in the first high-pressure side heat exchanger and a heat quantity to be extracted in the second high-pressure side heat exchanger. Thus, in the heat pump cycle, it can prevent the heat quantity to be extracted from the heat pump from decreasing and hence the coefficient of performance (COP) of the heat pump from becoming worse.
Preferably, a refrigerant temperature detecting device is disposed for detecting a refrigerant temperature between the first high-pressure side heat exchanger and the second high-pressure side heat exchanger, and the decompression unit controls the refrigerant pressure discharged from the compressor to become equal to or lower than a predetermined pressure based on the refrigerant temperature between the first high-pressure side heat exchanger and the second high-pressure side heat exchanger. Accordingly, the refrigerant pressure discharged from the compressor becomes higher, and the temperature of the first fluid can be sufficiently increased.
Preferably, fuel cells are disposed in the fluid circuit to generate electricity by chemically reacting hydrogen and oxygen, and the first high-pressure side heat exchanger is disposed in the fluid circuit to heat the first fluid flowing into the fuel cells. Therefore, the fuel cells can be heated using the heat pump, while the COP of the heat pump can be improved.